This invention relates to a method and an apparatus for controlling the temperature of the upper and lower portions of forming and stamping tools for the manufacture of thermoplastic containers. The apparatus is installed in the forming station of a thermoforming machine which draws a film web from a supply roll, heats the film web and thereafter sequentially deforms lengths of the film web in the forming station by means of the tool and by means of a differential pressure. Immediately following the deformation the same tool stamps out the container. After the tool is opened, the containers are conveyed away by means of systems known by themselves. The scrap web is wound or comminuted.
In thermoforming machines a shortening of the operational cycles (that is, an increase of the operational frequency) is sought for the purpose of increasing the output rate. The operational frequency is significantly affected by the cooling period necessary for the article made by the thermoforming machine. Such cooling period depends from several factors such as the type of the film, the film thickness, the film temperature during deformation and the shape of the formed article (container), particularly at its upper edge.
Since the film web is deep-drawn at temperatures in the order of magnitude of approximately 150.degree.-200.degree. C., the forming tool heats up during operation unless specifically cooled. During such heat-up the upper and lower portions of the tool will have different temperatures: normally a greater amount of heat is taken up by that tool part (usually the lower tool portion) which carries the mold proper. In practice it is therefore conventional to cool the lower tool part in order to achieve higher operational frequencies.
The critical zone during cooling of the article shaped in the thermoforming machine is mostly at the upper article edge which is thicker than the article wall and therefore needs a longer period for cooling. If the article (such as a container) is removed from the mold prematurely, its edge is distorted which is unacceptable if such an edge should, in use, perform a sealing function.
Because of the different temperature conditions in the upper and lower tool parts, conventionally two coolant circuits have to be controlled. As a rule, the thermoforming machine is associated with a refrigerating apparatus which delivers a coolant having the required low temperatures. Such a refrigerating apparatus operates usually with water as the coolant which is cooled to approximately 3.degree. C. and is then introduced into the tool halves. Dependent upon operational conditions, with the aid of such a system the tool temperature may be maintained between 20.degree. and 40.degree. C.
The requirements for ever-increasing output in thermoforming machines have led to arrangements in which the forming and stamping tools can, without difficulty, work on film widths of 800 mm and the containers can be made in groups, in which the containers are distributed in several rows. In general, 8 to 10 containers per row are feasible, dependent upon the diameter of the container.
Since the containers are stamped in the tool subsequent to the forming operation, the tools have to be high-precision parts. Cutting dies and matrix bores have to be in an accurate alignment during stamping; the clearance between the two (that is, the cutting play) is in the order of magnitude of 0.04 mm.
If the forming tool is of substantial width, the temperature difference between the upper and lower tool parts may become excessive, whereupon, due to the unlike expansion of the parts, the cutting play may disappear. As a result, the outer cutting dies in the row run onto the matrix edges which leads to damages resulting in high costs and interruption of production.
By throttling the flow of the individual coolant streams to the upper and lower portions of the tool, it would be, to be sure, feasible to obtain for the two tool portions predetermined separate temperatures set at respective temperature regulators. Such an arrangement requires throttle mechanisms in the supply conduit for the coolant leading to the upper and lower tool portions and a system which controls the throttles. It is a disadvantage of this type of coolant control that not the entire coolant volume capable to be supplied by the refrigerating apparatus is utilized. This then means that the tool is not brought to the lowest possible temperature. Such a lowest possible temperature, however, is desirable because it would result in an optimal cooling of the formed container, thus leading to a maximum output rate. It is noted that the value of such a temperature is usually not known and depends from many factors.